The spine is comprised of an intricate system of bones and assorted tissues that supports the body and provides protection of the central nervous system including the spinal cord and associated nerves. Within the spinal column are stacked a plurality of vertebrae separated from one another by an intervertebral disc that dampens and cushions the compressive forces exerted upon the spinal column. Located behind the series of alternating vertebrae and discs is the vertebral canal which contains the spinal cord and other associated nerves.
There are more than twenty vertebrae within the spinal column and they are categorized into one of four classifications: cervical, thoracic, lumbar or sacral. The upper seven vertebrae, including the first seven extending downward from the base of the skull are referred to as the cervical vertebrae. The next twelve extending downward from the cervical vertebrae and known as the thoracic vertebrae. Extending downwardly from the thoracic vertebrae are the five lumbar vertebrae. At the base of the spinal column is the sacral bone which also includes the coccyx. The structural and functional relationship of the vertebrae, discs, muscles, ligaments and nerves enables a healthy normal spinal column to move and articulate freely almost without limitation.
The spinal column is comprised of the vertebral body, the pedicle, the spinous process, the transverse process, the facet, the laminar arch, and the vertebral canal. The vertebral body is the generally cylindrically shaped weight bearing structure of the vertebra. The spinous process extends from the rear portion of the vertebra and the transverse processes extend from each side of each vertebra. Both the spinous process and the transverse process connect muscle tissue and ligaments to the spine. The vertebral canal is formed between the vertebral body and the lamina and houses the spinal cord therein. The pedicle is connected to the vertebral body and supports the lamina.
The spinal column may be subject to numerous abnormalities and disorders which can be caused by trauma, disease, or genetic defect such as ruptured or slipped discs, degenerative disc disease, fractured vertebrae as so forth. Such defects can result in conditions causing extreme pain a reduced or abnormal nerve function. These spinal abnormalities can potentially cause damage the nervous system and in particular the spinal cord and likewise impair the normal freedom of motion of the spinal column.
It is not uncommon to treat such abnormalities surgically by spinal fusion wherein one or more vertebral bodies are fused together. However, spinal fusion may limit the spinal cord's range of motion in rotation and lateral bending. In addition, spinal fusion may increase the stress placed upon non fused adjacent vertebral bodies thereby diminishing their structural integrity. Moreover, the fusion device or material may become dislodged move away from the area of implantation.
A wide variety of approaches have been in use to achieve spinal fusion by implanting artificial devices in or on the spinal column to result in immobilization. One approach utilizes an anterior implant where the implant is located on the anterior, or front portion, of the vertebral body. An anterior stabilization can include full or partial disc replacement by a rigid spacer that is approximately the size of the disc that has been removed. A different approach involves the utilization of a posterior implant. Posterior implants include rods that are attached to either the lamina or transverse process by hooks or by pedicle screws. Other posterior implants allow for flexible or dynamic stabilization using pedicle screws connected by rigid or flexible rod member. Prior art posterior pedicle screw based stabilization systems create forces that are often transferred to the anchored pedicle screws. Patients having a relatively brittle bone structure cannot withstand the magnitude of these forces without resulting in the failure of the anchoring system.